This invention relates to an optical recording disk, and more particularly, to an optical recording disk of the heat mode.
The optical recording disks have the feature that they are free of deterioration by abrasion because they are not in contact with writing and reading-out heads. Active research and development works have been focused on a variety of optical recording media.
Among others, optical recording disks of the heat mode have been of most interest because no development procedure in the dark room is necessary. The optical recording disks of the heat mode are optical recording disks utilizing recording light which is converted into heat. One typical example is a pit formation type optical recording disk in which writing is carried out by applying recording light such as a laser beam to the medium to locally melt or remove it to form a small dip called pit where information is recorded, and reading out is carried out by detecting the pit with the use of reading light.
In most pit formation type optical recording disks, particularly those compatible with a light source in the form of semiconductor laser having the benefit of reducing the size of the recording/reproducing system, a recording layer is formed of a tellurium base material. For several reasons including toxicity of tellurium materials, higher sensitivity, and manufacture cost reduction, an increasing number of proposals and reports were recently made for such recording media using organic materials, typically dyes as the recording layer (see Japanese patent application No. 59-19715).
The optical recording media having a recording layer of dyes are preferably designed as a so-called air sandwich structure in order to increase recording quantity. The air sandwich structure is usually produced by preparing a pair of optical recording segments each comprising a transparent substrate of resinous material having a recording layer thereon and joining together the segments such that the recording layers face one another via a predetermined space.
In joining together the optical recording segments to produce a disk of air sandwich structure, ring-shaped connecting members or spacers of resinous material are interposed between the segments along the central and peripheral edges and bonded or fused to the substrates of the segments. Undesirably, such an air sandwich structure is not sufficiently strong because of bonding or fusing of spacers. Cumbersome operations are necessary in that adhesive must be applied to either surface of a spacer, and three parts, two substrates and the spacer must be precisely aligned before the spacer is bonded. During bonding procedure, off-centering of the spacer at the center and offsetting of the substrates are likely to occur. Such misalignment would inconveniently raise a disturbance in dynamic balance during disk rotation. The procedure of fusing spacers is also inefficient.
In the prior art air sandwich structure, the interior space is kept air tight to the exterior. If optical recording disks are held under an ambient pressure different from that for manufacture, for example, during transportation by aircrafts, a pressure difference between the interior space and the ambient would induce strain in the substrates, which would often interfere with recording and reproducing operations.
To obviate these problems, there have been proposed the following techniques.
(a) Either outer or inner spacer is formed with a vent through which the interior space communicates with the ambient atmosphere (see Japanese Utility Model application Kokai No. 57-149535). PA0 (b) A substrate is formed with a vent through which the interior space communicates with the ambient atmosphere (see Japanese Utility Model application Kokai Nos. 58-105041 and 58-105042).
However, technique (a) has the drawback that since the spacer is as thin as about 0.2 to 0.8 mm, perforation of a vent port in the spacer is very difficult and the perforated spacer is mechanically weak. Technique (b) has the drawback that the presence of a vent port obstructs uniform coating in applying a recording layer of dye on a substrate as by spinner coating.
Most commercial optical recording disks generally bear an identification mark having the function of representing a trade mark or product grade. Such identification marks are given by printing marks or attaching labels onto disk substrates. The optical recording disks are under the trend that the area of a recording layer has as large a proportion as possible for increased recording capacity. This, in turn, limits the space available for marking. Further the conventional printing and labeling techniques have additional problems that marks might fade away or be removed and printing would leave stains. A printing or labeling step must be added to the conventional processsing line, requiring an additional installation investment.
The foregoing description is concerned with optical recording disks of the air sandwich structure capable of double-side recording, and the same applies to single-side recording disks of a similar structure in which one segment is merely comprised of a protective substrate.